As a method for removing nitrogen oxides from flue gases emitted from stationary emission sources such as thermal power plants, an ammonia-added selective catalytic reduction method has been applied, which uses a catalyst carrying V2O5, WO3, and MoO3 as active ingredients on a TiO2-based catalyst carrier. In this method, NH3 is added as a reductant to cause a NOx reduction reaction (4NO+4NH3+O2→4N2+6H2O) and V, W, and Mo supported on solid acid sites of TiO2 act as active sites.
When a V2O5/TiO2-based catalyst is used in the reduction of NOx from flue gases containing nitrogen oxides emitted in a case in which a coal or a heavy oil is used as the fuel, the activity of such a catalyst degrades over time. The degradation of the activity of a catalyst is due to a very small quantity of poisoning components contained in flue gases, such as As and Ca, accumulating on the catalyst and covering the catalyst active sites. In particular, As contained in flue gases is As2O3 with a high vapor pressure and is oxidized on the catalyst to be converted into As2O5 with a low vapor pressure (As2O3+O2→As2O5). This As2O5 reacts at the solid acid sites, and thus the catalyst activity is lost. Accordingly, when a low-grade coal including a large quantity of arsenic (As), which is a poisoning substance that may considerably affect the catalyst, is used as the fuel, the deterioration of the catalyst occurring due to the rapid loss of solid acid sites caused by As becomes highly problematic.
Incidentally, it has been blown that when an oxide is composited, the solid acid content of the complex oxide usually becomes higher than that of a single-element oxide, although this depends on the type of the element included in the oxide. Focusing on this finding, the inventors have made an attempt to improve the resistance of a catalyst to As by increasing the permissible dose of As by increasing the content of solid acid in a two-component system based on TiO2, such as a TiO2/SiO2 system (refer to Patent Literature 1, for example).